Since the 1850's, mineral wool for thermal and acoustic insulation has been produced from a wide variety of raw materials, including furnace slags from copper, lead and iron production. To make mineral wool, these materials are remelted in fuel-fired cupola furnaces which are primitive devices offering little quality control, substantial air pollution and, in recent years, high operating cost because of the steep rise in the cost of coke, their principal fuel.
Careful and detailed studies of the reactions in large cupolas such as an iron blast furnace, decades of effort to establish optimum levels for all its parameters, and enormous increases in size (several recently commissioned units exceed 10,000 tons of iron per day, or 14,000 pounds per minute) have resulted in predictable quality and reasonable economy.
By contrast, the small cupola furnaces (5 tons per hour) in use all over the world to produce molten non-metallics to be fiberized into mineral wool are small and inefficient. No economies of scale have been achieved, because mineral wool is bulky, and cannot be transported over great distances without absorbing the margin in freight costs. Further, the "spinners" used by most operators to fiberize the molten stream of slag discharging from the cupola are generally limited to 5 tons per hour per set, in present practice, and mounted one set per furnace, or "line."
As a result, the typical cupola currently in service to melt non-metallics for mineral wool is a water-cooled steel shell 6 to 7 feet in diameter and 15 to 25 feet high. It is by nature thermally inefficient, air polluting and high in operating cost. The quantities of particulate matter, sulphur and sulphur oxides in the top discharge of fume from the cupola require prohibitively high capital and maintenance costs to control, considering that only 5 tons per hour are melted.
The cupola's most important deficiency is its lack of control of the quality of the product. Residence time, in a molten state, of each increment of charge is very small, of the order of seconds in some cases or minutes at most. Modification of tapping temperature can only satisfactorily be achieved by charge additions, such as sand, to lower the melting point. Increase in melt rate can only be achieved by increasing the blast, with a consequent change in residence time and tapping temperature.
The ability of the spinning system to convert most of the cupola discharge into high quality product is a function of the surface tension of the molten stream, which in turn is affected by temperature, chemistry and viscosity. The inability of the cupola to control these variables results in poor average performance. Sometimes, when optimum fiberizing conditions are approached, a cupola/spinner combination converts a much higher percentage of its molten feed into high quality product, indicating that even modest control of the key melting variables will give significant improvement in yield.
Surface tension is a critical parameter in the fiberization process. The spinning wheel produces a plane sheet of liquid slag which is hit at right angles by a high velocity stream of air. The slag film is deflected and is subjected to aero-dynamic instabilities which develop into waves propagating with increased amplitude in more or less tangential orientation.
At the leading edge of the sheet, half or full wavelengths of the molten material are detached by the impact of the air blast and contract into ligaments under the influence of surface tension. What then happens to these ligaments, i.e. whether they are converted into useful fiber, or shot to be rejected, depends largely upon the temperature-viscosity relationship.
Since raw materials, particularly iron blast furnace slags, are in abundance as (mostly) waste matter, and mineral wool of good quality has high value as insulation, a number of attempts have been made over the last 20 years to find a more satisfactory melting method. These attempts have generally been based upon the use of an electric furnace for resistance, arc or induction melting of the charge, with a view to providing molten material which is controlled in terms of flow rate, temperature and composition, at a competitive cost.
Each of these attempts has failed, not because electric melting of slags is in itself particularly diffucult, but because its achievement in a controlled fashion with any conventional electric furnace has proved uneconomical.
The source energy used to melt a ton of blast furnace slag by means of a 5 ton per hour cupola may be shown to be about 7 million BTUs. Because of lack of control of the temperature, chemistry and rate of the cupola discharge, an average of 45 percent of this melted material is wasted as shot and tailings, so the source energy required for the melting of 1 ton of product is approximately 12.5 million BTUs. By contrast, under ideal conditions, the total heat required to raise 1 ton of iron blast furnace slag to tapping temperature is approximately 450 KWH, or 1.5 million BTUs. However, since the efficiency of a modern thermal power station is 37 percent at best, and transmission losses to the melting site will probably account for another 10 percent, the total source energy requirement to raise 1 ton of slag to tapping temperature is, under ideal conditions, 4.5 million BTUs. And therefore, in a conventional 5 ton per hour electric furnace of 70 percent overall thermal efficiency, source energy required is 6.4 million BTUs per ton melted. Assuming that the improvement in control of tapping temperature, chemistry and rate due to conventional electric melting provides an increase in useful mineral wool product from the present 55 percent to 65 percent, the net source energy requirement for this electric melter is 9.8 million BTUs per ton of product.
In summary, the source energy required per ton of mineral wool product is approximately 20 percent more for current cupola practice than it is for conventional electric melting.
Expressed in economic terms, at $170 per ton of coke, and an average cost in the United States of $0.028 per KWH (in 1979), the savings in energy cost indicated for conventional electric melting over cupola melting were approximately $10 per ton melted, or $18 per ton of product.
Unfortunately, these savings in energy cost are offset by the high cost of refractories in the conventional electric furnace, because molten slag and the presence of available oxygen will erode all known refractory lining systems, even carbon and graphite. Carbonaceous materials oxidized, or burn away increasingly rapidly as their temperatures rise above 500 degrees C. For example, industrial graphite loses 6 percent of its weight by oxidation when maintained at 600 degrees C. in air for only two and a half hours. The melting point of iron blast furnace slag, depending upon composition, is 1,370 to 1,540 degrees C.
Co-pending application Ser. No. 119,450 filed Feb. 7, 1980 now U.S. Pat. No. 4,389,724 (International Application No. PCT/US81/00129 published Aug. 20, 1981 as No. WO 81/02339) discloses an invention which substantially overcomes the above-described problems with the prior art systems. As disclosed therein, this is accomplished by constructing an electric melting furnace equipped for high integrity atmosphere control, thereby excluding atmospheric oxygen and permitting the use of carbonaceous materials as an economical refractory lining.
This fully enclosed furnace lends itself to thermal insulation of a very high order, permitting thermal efficiencies of 80 to 85 percent for a 5 ton furnace, with corresponding reductions in source energy requirements and operating cost.
The quantity of fume generated by a totally enclosed furnace, from which atmospheric air is excluded, is a small fraction of that resulting from the thousands of cubic feet of counterflow air blast needed for cupola operation. Consequently, fume handling for the new furnace is reduced to modest, relatively inexpensive proportions.
Charge increments are delivered through an atmosphere lock into a molten pool constituting approximately 1 hour of production. The resulting 30 to 60 minute residence time, in conjunction with fully variable energy input and charge and discharge rates, and controlled atmosphere, make the furnace inherently capable of very close control of tap temperature, chemistry and rate, permitting predictable surface tension and viscosity, and corresponding improvements in product quality and yield.
The furnace disclosed in the co-pending application also accepts and recycles the rejected shot and tailings which cannot be utilized by the cupola, thereby permitting significant savings in raw material and waste handling costs.
The cumulative effect of the foregoing advantages are substantial savings in source energy and operating cost. With reasonable refractory life, a furnace efficiency of 85 percent, a spinner yield of 75 percent and full recycling of shot and tailings, the source energy required per ton of product drops from 12.5 million BTUs in the cupola to 7 million BTUs, and operating cost drops by more than $40 per ton of product, using 1979 figures.